gem-bearing pegmatites: a revie articles...gem-bearing pegmtites : a review by lames e. shigley and...

GEM-BEARING PEGmTITES : A REVIEW By lames E. Shigley and Anthony R, Kampf

Many of the important gem ~ninerals seen on today's marliet-aquamarine, tourma- h e , and topaz, among others-come from an unzisual type of rock lznown as a pegmatite. Gem-bearing pegmatites are crystalline igneous rocks that are distin- guished by their large-size crystals, concentrations o f certain chemical elements otherwise rare in the earth's crust, and various ur jus~~al minerals. l'egmatites are typically rather small bodies of roc)< that are found in particular geologic environments; the gem minerals occur jn open cavities or "poclzets" within the pegmatite. This article surveys our current understanding of pegmatites, beginning with a brief de- scription o f their characteristics and fol- lowing with a disc~ission of the occurrence of gem minerals in them. The article con- cludes wi th a summary o f the specific conditions necessary d ~ ~ r i n g pegmatite formation for the crystall~zaiion of abundant gem minerals.

ABOUT THE AUTHORS

Dr. Shigley is research scientisl in /he Depar/men/ of Research, Gemologica/ lns/ilute of America, Santa Mon~ca, California; and Dr. Kampfis curator of gems and minerals, Los Angeles Counly Mu- seum of Natural History, Los Angeles, California.

0 7 984 Gemological lnsti/ule of Amer~ca

A quamarine, tourmaline, topaz, lzunzite, morganite-these are but a few of the gemstones f o ~ ~ n d

in the remarlzable mineral deposits that geologists call pegmatites (figure 1). Of the many different lzinds of roclz exposed at the earth's surface, pegmatites contain the greatest ab~~ndance and variety of gem minerals. Pegmatite deposits in various parts of the world have yielded spec- tacular crystals of gem tourmaline (figure 2)) topaz (figure 31, and beryl (figure 41, as well as a host of other minerals occasionally used as gems (see tables 1 and 2). Most of these minerals are only rarely found in other geologic environments in crystals suitable for faceting, In addition, pegmatites are a major source of certain rare elements of great economic importance.

This article briefly summarizes current knowledge concerning the nature and formation of these fascinating roclzs and the occurrence of gem minerals within them. Individual pegmatite localities are not discussed in detail here. Rather, a broad overview is presented to demonstrate the remarlzable similarities between gem-producing pegmatites in diverse parts of the world. Armed with this perspective, the reader should be able to better appreciate detailed reports of pegmatite occurrences such as those in Brazil that are discussed elsewhere in this issue.

WHAT IS A PEGMATITE? The famous mineralogist Hafly first used the word pegmatite in the early 1800s to refer to a roclz with a patterned geometric intergrowth of feldspar and quartz (now commonly termed graphic granite). Today it is applied to any crystalline roclz that is, at least in part, ex- tremely coarse grained. The term pegmatjte, then, primarily refers to the texture of a roclz, that is, the size, shape, and arrangement of mineral grains. In practice, it can be applied to a wide range of roclzs of igneous or metamorphic origin that exhibit large crystals.

64 Gem-Bearing Pegmatites - GEMS & GEMOLOGY Summer 1984

In reality! the vast majority of pegmatites are found to be chemically and mineralogically similar to ordinary granites andl hence! are called "granitic1' pegmatites (as opposed to "gabbroicl' pegmatites' lfsyeniticll pegmatites' etc.). Because gem minerals arel for the most part) found only in granitic pegmatites! these will be the focal point of the ensuing discussion.

Texture. Although pegmatites are commonly thought of as very coarse-grained roclzsf they ac- tually vary considerably in grain size. This varia- bility is important in distinguishing pegmatites from most other crystalline rocks. For example!

Gem-Bearing Pegmatites

Figure 1. A selection of gemstones of pegmatite origin ranging from 5.87 to 11.25 ct. Minerals shown (from left to right, and from front to baclz) are us follows: tourmuline (elbaite), garnet (spessartine), chrysoberyl; topaz, feldspar (or~hocluse), beryl (morgunite); beryl (aquumarine), tourmaline (elbaite), tourmaline (elbaite); beryl (heliodor), spodumene (kunzite). Photo by Tino Hammid.

the mineral grains in a granite are quite uniform in size and only rarely exceed several millimeters in diameter. Those in a pegmatite are usually several centimeters across on average! but they can range from millimeters to meters in diameter (figure 5). Typically there is an increase in crystal size from the outer margins toward the interior of the pegmatite body.

The largest crystals ever found have come from pegmatites (seel for example! Jahns! 1953; Rickwood! 198 1; Sinkankas' 198 1). Outstanding examples include a 14-m-long spodumene crystal from the Etta mine in the Black Hills of South Dakota and an 18-m-long beryl crystal from a

GEMS & GEMOLOGY Summer 1984

pegmatite at Malakialina! Madagascar. The famous Harding pegmatite in New Mexico contains spodumene crystals that are 5 m long (figure 6 ) . Unfortunatelyl these and most other giant crystals are not of gem quality. There are! however, excep- tions: for instance! a 300-lzg transparent gem topaz from a pegmatite in Minas Gerais, Brazil! is now on display in the American Museum of Natural His- tory in New York.

Mineralogy and Chemistry. The minerals that occur in any rock depend on the rock's overall chemistry and the pressure and temperature conditions under which it formed. Most granitic pegmatites are composed of the same minerals found in ordinary granites! that isJ feldspars (microclinel plagio~lase)~ quartz! micas (muscovite, biotite)! and on occasion some common accessory minerals (table 3; also figure 5). As such

Figure 2. Multicolored crystal of to~~rmal ine wi th albite feldspar and quartz from a pegmatite in Afghanistan (14 x 9 cm), Photo 0 Harold d Erica Van Pelt. Specimen courtesy of David Wilber.

these pegmatites have generally been of limited economicl scientific! or gemologic interest. A small percentage of pegmatites! however! contain additional minerals! such as beryl and tourmaline) which incorporate certain rare elements. This latter group of granitic pegmatites, which are the source of most gem rough! have been the principal objects of pegmatite exploration and mining.

Establishing the chemical composition of a rock provides important clues regarding its origin! geologic history, and relationships to other rock types. Relatively fine-grained roclzsl such as granites! are easily sampled and analyzed to determine overall chemical composition; however, this is not generally the case for pegmatites. Their coarse and variable grain size) nonuniform distribution of minerals! and often poor surface exposure pose serious obstacles to accurate chemical analysis. Nevertheless! painstalzing work at a number of

66 Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984

crystals wjth smolzy quartz from a pegmatite in the Ural Mountains of the Soviet Union (9 cm wide). Photo 0 Harold etl Erica Van Pelt. Specimen courtesy of David Wilber.

Figure 4. A superb crydt.01 of morganite

beryl with albite feldspar from ;he White Q ~ ~ e e n r ~ ~ i n e , Pala, California

(4 x 5 cm). Photo 0 Harold d Erica Vun Pelt.

Table 1. Occurrence of gem minerals in pegmatites. m

Relative Pegmalites the Mineral abundancea major source?

Common Feldspar (gem varieties A no

orthoclase, amazonite, moonstone)

Quartz (gem varieties rock crystal, amethyst, smoky, rose, citrine)

Unusual (containing rare elements such as Li, Be, B, P, F, Cs, etc.)

Apatite Amblygonite Beryl (gem varieties

aquamarine, morganite, heliodor, goshenite)

Beryllonite Brazilianite Chrysoberyl Danburite Euclase Garnet (spessartine) Hambergite Herderile Lepidolite Petalite Phenakite Pollucile Spodumene (gem varieties

kunzile, hiddenite) Topaz Tourmaline (elbaite-gem

varieties rubellite, achroite, indicolite; liddicoatite)

aRelatiave abundance in granitic pegmafiles: A =abundant and widespread; C = common or locally abundant; R = rare or uncommon; VR = very rare.

localities has yielded meaningful estimates of the chemical composition of granitic pegmatites (see table 3).

Figure 5. Small pegmatite body cutting through granite at the Fletcher stone quarry near Westford, Massachusetts. Both the pegmatite and enclosing granite are composed of feldspars, mica, and qLlartz, but within [he peg~nu~ite the crystals are m ~ ~ c h larger, In addition, the cryslals near the center of the pegmatite are larger ~ h a n those along the outer margins. Pho~o by Richard H. Iahns.

Common granitic pegmatites show little devi- ation from typical granite chemistry (compare columns 1 and 2 in table 3). In contrast, those that contain small amounts of unusual minerals exhibit marlzed enrichments in a variety of rare elements (table 3, column 3). Lithium, berylliuml boron, and fluorine, in particular, are essential constituents in several important gem minerals. Despite their small total amounts (seldom over 2 wt. expressed as oxides), the concentrations of these and other rare elements in some granitic

TABLE 2. Some of the more important g e m pegmatite regions.

Region

-- --

Tourmaline Beryl Spodumene Topaz Quartz Garnet

Afghanislanlpakistan Brazil Madagascar Mozambique Namibia Soviet Union

Ural Mountains Transbaikalia Ukraine

East Africa United States

New England Colorado California


TABLE 3. Chemical and mineralogical comparison of granite and granitic pegmatites,

Component/ Common Gem-bearing phase Granitea pegmatitec

Chemistry Si02 A1203 FeO + ~ e i 0 ; Ti02 MnO "20 MgO CaO Na20 K20 Li20 P2Â° F B2Â° Be0 Rb20 + Cs2O

Total

Mineralogy Major phases

Minor phases

99.86

Microcline Quartz Plagioclase Muscovite Biotite

- - - 0.28

0.6 0.39 trace

0.3 1.36 4.6 4.45 4.2 2.85 - 1.49

0.3 0.07 0.1 0.1 1 - 0.18

trace trace - trace

Microcline Quartz Albite Muscovite

Beryl Tourmaline Apatite Garnet

Microcline Quartz Albite Muscovite

Beryl Tourmaline Apatite Garnet Spodurnene Rhodizite Lepidolite Hambergite Danburite

'Westerly granite, Westerly, Rhode Island (Turtle and Bowen, 1958). ^~iamond Mica pegmatite, Keystone, South Dakota (Norton, 1970). cGem-bearingpegmatite, Manjaka, Madagascar (Schneiderhdhn, 1961).

Figure 6. View of the Haiding pegmatite in Taos County, New Mexico. Several internal zones are visible as colored band's with differing texture and mineral content. This large, complex pegmatite, approximately 80 m thick, is rich in lithium minerals and rare elements, but apparently crystallized at too great a depth in the crust to allow for the formation of gem-bearing pockets. The elongate, ligh t-colored crystals are common spodumene reaching 5 m in length. Photo by Richard H. Jahns.

pegmatites can exceed by several orders of magni- tude the amounts found in other rocks. In fact, the mining of pegmatites for rare elements, or even for the common pegmatite minerals, has at times been of far greater economic importance than the mining of pegmatites for gemstones. For instance, the Harding pegmatite (figure 6) was an important wartime source of beryl for beryllium, of lepidolite and spodumene for lithium, and of microlite for tantalum.

DESCRIPTIVE CLASSIFICATION Efforts over the past century to better understand granitic pegmatites and their distinctive features have led to numerous attempts to classify them according to some logical framework. Most early schemes were descriptive in nature, and were based on various observable features such as shape or key minerals (see Landes, 1933; Jahns, 1955; Vlasov, 1961; Cerny, 1982). One of the most widely accepted of these descriptive classifica- tions was summarized by Heinrich (1956). This scheme, which is still useful today, divides granitic pegmatites into three types on the basis of internal

1.

2.

structure: Simple (figures 5 and 7a)-lack any segregation of minerals; may or may not display a systematic variation in texture; consist mostly of common silicate minerals with accessory minerals on some occasions. Zoned (figures 7b, 8, and 9)-possess distinct internal zones of contrasting mineral content and texture; often larger than the simple types; consist of common silicate

Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984

Figure 7. Idealized diagram showing internal structural relationships in

simple (a), zoned (b), complex (c), pegmatites.

Artwork by Christine Wilson; adapted from

Johns ( 1 953).

Granitic host rock - No internal zones: quartz-microcline-muscovite

Wall zone: quartz-plagioclase-microcline-muscovite-biotite

Intermedate zone: microcline-plagioclase-quartz-muscovite

Core zone: quartz

- Core zone: quartz-spodumene

I Gem pockets-secondary replacement minerals

minerals and various accessory species; minerals become more coarse grained toward the interior of the pegmatite.

3. Complex (figures 6 and 7c)-similar to the zoned types except also exhibit extensive mineral alteration and replacement; often contain high concentrations of rare elements and unusual minerals.

Simple pegmatites are by far the most numerous, while complex pegmatites are the least common but of greatest interest to the geologist and miner. Giant crystals, rare elements, and gem minerals are generally restricted to zoned and complex pegmatites.

Minerals found in the latter two types of pegmatites are arranged in layers or zones. As il- lustrated in figures 8 and 9, in an ideal situation these shell-like zones are concentrically disposed around an innermost core and tend to follow the exterior shape of the pegmatite body. Although the internal structure of many zoned pegmatites is rarely so uniform, and may be quite complicated,

fieldwork at numerous localities has documented a basic internal arrangement of minerals in pegrnatites that is generalized in table 4. In the field, recognition of this mineral arrangement in a pegmatite is of enormous help both to the student of pegmatites and to the miner (Sinlzankas, 1970).

GENETIC CLASSIFICATION Building on the earlier studies, more recent attempts to classify granitic pegmatites have em- phasized the geologic environment of pegmatite formation rather than descriptive details. For example, an alternative scheme proposed by Ginzburg et al. ( 1979; summarized in Cernv, 1982) uses the conditions present at various levels in the earth's crust to help explain observed differences in the nature of pegmatites. They recognized four classes of pegmatites:

1. Those formed at very great depth (more than 11 km), which are generally unzoned and possess little economic mineralization other than occasional concentrations of

70 Gem-Bearing Pegmatites GEMS &. GEMOLOGY Summer 1984

Figure 8. View of the Oregon NO. 2%

uranium, thorium, and rare-earth elements.

2. Those formed at great depth (approximately 7- 11 lzrn), which may be zoned, are generally rich in mica, but have few rare elements.

3. Those formed at intermediate depth (approximately 3.5-7 lzrn), which are often zoked, may contain small crystal-lined pockets, and possess a number of rare elements.

4. Those formed at shallow depth (less than 3.5 lzrn), which are zoned, and which sometimes contain rare elements and gem pockets.

Ginzburg et al. further contend that at very great depth pegmatites are formed largely through the partial melting or metamorphic recrystalliza- tion of existing roclzs essentially in place, while at lesser depths pegmatite formation becomes more and more an igneous process involving the injec- tion of a magma (molten rock) and its subsequent crystallization.

OCCURRENCE Geologic Setting. Compared to most other rock types, pegmatites are relatively rare, yet they are widely scattered in the earth's crust and can be locally abundant. Pegmatites tend to be most common in particular geologic settings, generally where igneous or metamorphic rocks are exposed at the earth's surface (Jahns, 1955; Schneiderhohn, 1961; terny, 1982). Those pegmatites formed at very great depths are usually found in metamorphosed roclzs that comprise the ancient cores of continents (e.g., the uranium-rare-earth peg-

pegmatite in the South Platte pegmatite

I district, Jefferson County, Colorado. This zoned granitic pegmatite is a vertical, column-shooed body with an exceptionally well-developed internal arrangement of minerals. The feldspar- rich intermediate zones were removed during mining operations, while the central quartz core was left standing. The pegmatite body is approximately 7 0 m in diameter. Photo by William B. Simmons, /r.

Granitic host rock

Wall zone: quartz-microcline-biotite

Intermediate zone: microcline

1 Core zone: quartz I

Figure 9. Idealized block diagram of a zoned granitic pegmatite similar to the one shown in figure 8. Note the horizontal and vertical zone structure and the apparent outcrop pattern resulting from the different levels of exposure possible. Artwork by Christine Wilson; adapted from Simmons and Heinrich (1 980).

matites near Bancroft, Ontario). Those formed at deep and intermediate depths occur in folded and metamorphosed rocks in mountain belts (e.g., the mica pegmatites in the Soviet Union, and the beryl-spodumene pegmatites of the Black Hills, South Dakota). The shallow-depth pegmatites are

Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 71

associated with large, buried masses of intrusive igneous rocks, known as plutons or batholiths, that frequently underlie mountainous areas (e.g., the gem-bearing pegmatites of southern Califor- nia, of the Hindu Rush region of Afghanistan, and of Minas Gerais, Brazil). Pegmatites in each of these depth-of-formation categories gradually become exposed at the surface over geologic time by either erosion or large-scale mountain uplift, and thereby become accessible.

Genetic Relationships. The close petrologic rela- tionship that is sometimes apparent between pegmatites and plutonic igneous rocks has been taken as evidence that most pegmatites them- selves result from the crystallization of silicate magmas. However, this connection is most obvi- ous for the shallow pegmatites. As depth increases, metamorphic processes seem to play a greater role in pegmatite formation.

Age. The ages of pegmatites span much of the geologic time scale from 3.9 billion to less than 100

million years ago. As might be expected from their frequent occurrence in mountainous regions, pegmatites can often be correlated in age with the corresponding orogenic (mountain-building) periods.

Size. In general, pegmatites are rather small rock bodies, although quite large ones do occur. Out- crops of pegmatites have been observed to range from centimeters to meters in minimum dimension and up to several kilometers in maximum dimension (e.g., compare figures 5 and 6). Typi- cally, pegmatite bodies are completely enclosed by other kinds of rocks. The actual dimensions of pegmatites are often difficult to estimate, not only because they are frequently irregular in shape, but also because very little of the pegmatite is exposed on the surface.

Shape. Pegmatites are among the least regular and most varied in shape of all rock bodies. This wide diversity can be attributed to a number of factors, including the depth of formation, the mechanical

TABLE 4. Some generalized features of internally zoned granitic pegmatitesa

Zone Thickness Mineralogy

Texture (accessory phases) Other comments

Border Usually a few centimeters, but sometimes thicker

Wall Usually on the order of several meters

Intermediate Each zone may reach (possibly several meters in thick- several ness depending on the zones) size and shape of the

pegmatite

Core Up to several meters in thickness depending on the size and shape of the pegmatite

Generally coarser than border zone

Progressively coarser grain size proceeding inward; some giant crystals; inner~riost zones may contain some pocket-rich areas

Variable-may contain both coarse- and fine- grained material; some giant crystals; may include pocket-rich areas

Plagioclase-quartz- muscovite (garnet, tourmaline, other phases) Plagioclase- microcline-quartz (muscovite, beryl, tourmaline, garnet, other phases) Microcline-quartz- spodumene-amblygonite- muscovite-plagioclase (tourmaline, phosphate minerals, beryl, other phases)

Quartz-spodumene- microcline (tourmaline, lepidolite, beryl, topaz, gem minerals, other phases)

May or may' not have sharp contacts with the surrounding host rocks

May not be continuous or of uniform thickness around the entire pegmatite

May consist of a number of distinct zones of differing mineralogy; each may or may not completely enclose the central core; intermediate zones contain the giant crystals and comprise the bulk of the pegmatite; unusual minerals often concentrated toward core Core zone may be composed of several segments; gem pockets often located on the contact between the core and the enclosing intermediate zones

aAdapted from Cameron el a/. (1949) and Norton (1983).


Figure 10. Portion of an open gem pocket in the famous Himalaya pegmatite of Mesa Grands, California, with multicolored crystals of gem tourmaline up to several centimeters in length along with pale crystals of feldspar and quartz. Photo by Michael Havstad.

properties of the host rock at that depth, and the tectonic and metamorphic processes that took place at the time of formation. Shallow pegmatites often fordsheet-like dikes, veins, or lenses that occur along faults or fractures in pre-existing host rocks. Deeper pegmatites, on the other hand, tend to be elliptical or ovoid as a result of the more plastic character of the enclosing rocks at depth.

OCCURRENCE OF GEMSTONES Gemstones never constitute more than a small portion of any pegmatite body. Certain of the more common gem minerals, such as tourmaline, beryl, and spodumene, can occur as giant crystals in the intermediate or core zones of pegmatites. How- ever, such large crystals are almost always highly fractured and clouded with inclusions, and conse- quently are of little or no gem value. The smaller, transparent crystals of these and other gem minerals are generally found only in pegmatite cavities, or "pockets."

Pockets are irregular openings in otherwise solid pegmatite (see figure 10). As found today, these pockets are usually filled partly or completely with clay, but during pegmatite crystallization they provided the necessary open space into which crystals could grow unimpeded, thereby attaining a very high degree of internal and external perfec- tion.

Pockets are most common in the complex,


internally zoned pegmatites, but even here they are infrequent. Although pocket-bearing granitic pegmatites are widespread in such regions as Minas Gerais, Brazil, and southern California, Jahns (1955) suggested that these cavities probably occur in less than one percent of all known pegmatites. Furthermore, few pocket-bearing pegmatites have the rare elements necessary for the formation of gem minerals in high-quality transparent crystals (figure 10). In fact, most contain only crystals of the basic constituents of the pegmatite itself, namely quartz, feldspars, micas, and schorl.

Even within the same pegmatite body, pockets can vary greatly in size and shape. Most are less than several centimeters in diameter, but a few several meters across have been reported. Never- theless, the total volume of pockets is usually triv- ial in comparison to that of the enclosing pegmatite. There seems to be no particular rela- tionship between the dimensions of the pegmatite and the number, size, or shape of its pockets. Even within the same pegmatite region, some pegmatites are remarkably rich in pockets (e.g., the Himalaya pegmatite shown in figure 11)) while others have few if any.

Pockets are usually found within the central core or along the margins between the core zone and the enclosing intermediate zones. The mineral content from one pocket to another, even in a given pegmatite, is likely to vary considerably.

Figure 12, a diagram of an actual gem pocket found in a granitic pegmatite in southern Califor- nia, illustrates several important characteristics of gem pockets. Pocket crystals are firmly rooted in the surrounding massive pegmatite, but in the open space of the pocket they are able to grow freely and thereby develop regular crystal faces. Crystals of feldspar and quartz are usually larger and more abundant than those of any gem minerals that may be present. Pocket crystals are often distributed nonuniformly, as is the case with the pocket in figure 12, where tourmaline was only found in place on the roof of the cavity. In some instances these crystals are etched or corroded as a result of chemical attack (see figure 13) and may exhibit replacement by secondary minerals. Bro- ken crystals of feldspar, quartz, tourmaline, and other minerals that once grew from the walls are sometimes found scattered about in the pocket, usually embedded in clays that formed after the crystals had finished growing. Few gem species are


Figure 1 1. Underground mine view o f the Himalaya pegmatite. The pegmatite, only about 1 m thick, is remarkably rich i n gem pockets and has been a noted producer of gem tourmaline. The pegmatite consists o f tan and grayish feldspars, grayish quartz, pink lepidolite, and blaclz schorl. The light-colored areas along the central portion of the pegmatite arenewly exposed but still intact gem pockets. Within the pockets, crystals of gem tourmaline are embedded i n clays and other secondary alteration minerals. Photo b y Michael Havstad.

found in any one pocket-in this case tourmaline was the only gem mineral present.

Some gem minerals exhibit compositional zonation to a varying degree within individual crystals. In an extreme example, such as tourmaline, this zonation is reflected in color zoning. That is, tourmaline crystals may have black, opaque (schorl) roots in the solid pegmatite and become pink, green, blue, etc. (elbaite) as they approach

and project into a pocket. Miners often use the color changes in tourma-

line embedded in the massive pegmatite as an in- dication that a pocket area may be close by. Other indicators are the presence of lepidolite, the increased transparency of quartz, the blaclz staining of the pegmatite by manganese oxides, extensive rock alteration, and the presence of clays. Miners quickly learn, however, that each pegmatite is

Figure 12. Diagram of a mapped, vertical cross section through a small, tourmaline-bearing gem pocket in the Stewart pegmatite, Pala, California. Crystal fragments of quartz and gem tourmaline, broken during the final stages of pocket formation, are embedded i n the pocket-filling clay material. Artwork by Christine Wilson; adapted from Johns (1979).

1 Microcline Feldspar

1 Albite Feldspar

Quartz

1 Muscovite

Lepidolite

I] Pocket clay with numerous small crystal fragments

2.5cm 1 1 Tourmaline

Massive Pegmatite surrounding pocket

74 Gem-Bearing Pegmatite8 GEMS & GEMOLOGY Summer 1984

unique, with its own set of pocket indicators. They also learn that, despite careful attention to these clues, the location of productive gem pockets is still largely a matter of luck and hard work.

THE GENESIS OF GEM PEGMATITES Although in the past there has been considerable disagreement among geologists regarding the origin of granitic pegmatites, there is a general con- sensus today that at least the shallower ones formed by crystallization from a magma. Numer- ous theories have been proposed to explain the formation of pegmatites via magmatic crystallization (see Landes, 1933; Jahns, 19551, but perhaps the most widely accepted general model is that of Jahns and Burnham (1969; also see Jahns, 1953, 1979, 1982). Their model is based on the work of many early investigators, notably Fersman ( 193 1). The key points of this genetic model relevant to internally zoned granitic pegmatites containing gem minerals are described below.

Starting Materials. The starting material for a gem-beaii& pegmatite is a volatile- and rare element-!rich silicate magma derived from the final stages of crystallization of certain granitic magmas. 'Water is the most important volatile constituent of this pegmatite magma; however, other volatiles-such as fluorine, boron, lithium, carbon dioxide, and/or phosphorus-may be present. The rare elements include beryllium, cesium, niobium, tantalum, and tin, among others. Both groups of constituents may be present at much higher levels in pegmatite magmas than in the parent granitic magmas.

Emplacement and Initial Crystallization. This pegmatite magma is injected into fractured rock in the upper portion of the crust; as the temperature falls, the magma begins to crystallize. Mineral formation begins at the outer margin of the pegmatite magma chamber at temperatures somewhat below 1000Â°C Plagioclase feldspar, quartz, and muscovite mica crystallize first as the fine-grained border zone. Later, these are joined in the coarser-grained intermediate zones by microcline feldspar and additional minerals, such as common spodumene and beryl.

Concentration of Volatiles. The major chemical elements of the pegmatite magma determine the nature of the abundant, first-formed minerals such


Figure 13. This crystal of gem green beryl from the Ukraine region of the Soviet Union (9 x 17.5 cm) displays intriguing surface features caused by chemical etching that took place subsequent to i ts formation i n a pegmatite gem pocket. Photo 0 Harold o) Erica Van Pelt.

as feldspars, quartz, and micas. Because of their chemical and structural makeup, the early-formed minerals cannot, for the most part, incorporate the volatiles or rare elements present in the magma. As a result, these components are preferentially retained in the magma, where they become progressively concentrated. As crystallization continues with further cooling, the water content of the magma eventually reaches a saturation level. At this point, an aqueous fluid, rich in volatiles and certain rare elements, separates from the remaining pegmatite magma at between 750Â and 650Â°C

Nature of Aqueous Fluid. The physical properties of this aqueous fluid are markedly different from


those of the magma and, therefore, the fluid greatly affects the subsequent crystallization of pegmatite minerals. The much lower viscosity of the aqueous fluid permits the rapid transport of chemical nutrients to the growing crystals, thereby promot- ing their growth in the innermost zones of the pegmatite. Its greater concentration of volatiles and many of the rare elements contributes to the partitioning of elements between magma and fluid, and thereby to the segregation of minerals in separate zones. In addition, vertical segregation of minerals has been attributed to the rise of the less-dense fluid within the pegmatite magma chamber. The aqueous fluid seems, in general, to be a superior solvent. Minerals crystallize from it at lower temperatures, and can grow to greater size, than from the magma. The fluid also tends to redissolve some earlier-formed minerals with which i t comes in contact. This fluid is probably responsible for much of the secondary mineral replacement observed in complex pegmatites.

Intermediate Stages of Crystallization. The volatile-rich, aqueous fluid continues to exsolve from the magma as crystallization proceeds, and minerals in the inner zones of the pegmatite form in the presence of both the fluid and the magma. The last remaining magma eventually disappears at temperatures between 600Â and 500Â°C The innermost portions of the pegmatite are now occu- pied by large crystals of feldspar and quartz (and possibly common spodumene or beryl) with a few isolated, intervening "pockets" of trapped fluid.

Formation of Pocket Crystals. The formation of minerals in open pockets is the final stage in the primary crystallization of the pegmatite. With decreasing temperature (600' to 400Â°C and rising internal pressure (resulting from the release of vol- a t i le~) , crystallization continues from the fluid. Euhedral crystals of various minerals are able to form from the fluid within the open space of the pockets. At this stage, the concentrations of certain rare elements may reach sufficiently high levels for the crystallization of corresponding unusual minerals. In some instances, non-gem-quality crystals projecting into the pocket continue to grow and attain a more flawless, gemmy termina- tion. This continued mineral growth is accompa- nied by changes in the chemical composition of the fluid which, in turn, are reflected in corresponding changes in some minerals (i.e., as exem- plified by the color zonation of tourmaline). The

occurrence of abundant liquid inclusions in many pocket crystals (and the gems faceted from them) attests to their growth from an aqueous fluid. The final temperature for mineral crystallization from this fluid may be as low as 250Â°C

Evolution of the Pocket. Unfortunately, the chang- ing fluid chemistry, coupled with decreasing temperature and increasing pressure, can eventually lead to the destruction of many pocket crystals. In addition, earlier-formed minerals may become un- stable in contact with this highly reactive fluid. As a result, some pockets are found to contain only the remnants of what may have been gem-quality crystals altered to secondary minerals such as lepidolite mica and montmorillonite clay. Most of those pockets that ultimately produce gem minerals are thought by some to have undergone a final, very important step. This step involves the leakage of volatile fluids through breaks in the pocket walls that may have resulted from one or more of a number of possible mechanisms:

1. Increase in fluid pressure within the pockets that eventually exceeds the confining pressure or strength of the surrounding massive pegmatite.

2. Cooling and contraction of the pegmatite body.

3. Earth movements in the vicinity of the pegmatite body.

If the leakage of volatiles is gradual, the pocket crystals will remain intact. Unfortunately, evidence indicates that the pocket fluid often escapes rapidly, resulting in a very sudden drop in pressure and a consequent dramatic decrease in temperature. The resultant "thermal shock" is thought to be responsible for much of the internal fracturing often observed in gem crystals, as well as for the shattered fragments of crystals found on the floors of many gem pockets or embedded in the pocket clay. The fluid lost from the pocket is injected into fractures in the surrounding pegmatite where i t results in the replacement of earlier-formed minerals.

Depth of formation is an important factor in determining whether pocket rupture will occur. It also seems to be a factor in determining whether pockets will form at all, because pockets are only found in the shallow and intermediate-depth pegmatites. It has been suggested that the separate, less dense, aqueous fluid phase, thought to be re- quired for pocket formation, is only able to exsolve


from the magma under the lower confining pres- sures experienced by pegmatites formed at shallower depths. This would explain the apparent lack of pockets in deeper pegmatites and the ab- sence of pockets altogether from pegmatites in many parts of the world.

SUMMARY Pegmatites are among the most geologically inter- esting and gemologically important types of rocks exposed at the earth's surface. They are crystalline rocks that are often characterized by highly variable texture, giant-size crystals, unusual minerals, and concentrations of rare elements. Pegmatites originate from residual magmas derived by partial melting of crustal rocks or as products of the final stages of igneous crystallization. Crystallization of these magmas, which are sometimes rich in volatiles and rare elements, gives rise to the distinctive mineral content and textural features of peg-

mat i t e~ . A small percentage of pegmatites contain well-formed crystals of gem minerals in pockets in their interiors.

With the increased demand for colored gemstones, the mining of pegmatites for gem material will continue to be an activity of small scale but great economic importance. This demand is lead- ing to accelerated exploration of pegmatite regions for new sources not only in long-established areas such as Brazil and southern California, but also in newly discovered or recently accessible areas such as East Africa, Madagascar, and Afghanistan. Gem-bearing pegmatites are most likely to be found in geologic environments where the pegmatite magmas crystallized at shallow crustal depths. Pegmatites that form in other geologic settings hold less promise as sources of gemstones because physical and chemical conditions in these instances seem to preclude the formation of open pockets and, therefore, gem crystals.

Cameron E.N., Jahns R.H., McNair A.M., Page L.R. (1949) In- terrial Structure of Granitic Pegmatites. Monograph 2, Eco- nomic Geology, Urbana, IL.

t e rn$ P., Ed. (1982) Granitic Pegmatites in Science and Indus- try. Short Course Handbook, Vol. 8, Mineralogical Associa- tion of Canada, Winnipeg.

Fersman A.E. (1931) Les Pegmatites, leur Importance Scien- tifiqiie et Practique. Akademiya Nauk SSSR, Leningrad. Translated from Russian into French under the direction of R. du Trieu cle Terdonck and J. Thoreau, University de Louvain, Louvain, Belgium, 1952.

Ginzburg A.I., Timofeyev I.N., Feldman L.G. (1979) Principles of Geology of Granitic Pegmatites (in Russian). Nedra, Moscow (summarized in Cerny, 1982).

Heinrich E.W. (1956) Radioactive pegmatite deposits-how to know them. Canadian Mining Journal, Vol. 77, No. 4, pp. 69-72, 100.

Jahns R.H. (1953) The genesis of pegmatites. 1. Occurrence and origin of giant crystals. American Mineralogist, Vol. 38, pp. 563-598.

Jahns R.H. (1955) The study of pegmatites. Economic Geology, Fiftieth Anniversary Volume, pp. 1025- 1 130.

Jahns R.H. (1979) Gem-bearing pegmatites in San Diego County. In P.L. Abbott and V.R. Todd, Eds., Mesozoic Crys- talline Rocks: Peninsular Ranges Batholith and Pegmatites, Point Sal Ophiolite. Department of Geological Sciences, San Diego State University, San Diego, CA, DD. 1-38. r r - - -

Jahns R.H. (1982) Internal evolution of granitic pegmatite~. In P. Cern*, Ed., Granitic Peematites in Science and Industry, Short course Handbook, V O I . 8, Mineralogical ~ssoc ia t ion of Canada, Winnipeg, pp. 293-327.

Jahns R.H., Burnham C.W. (1969) Experimental studies of pegmatite genesis. I. A model for the derivation and crystallization of granitic pegmatites. Econornic Geology, Vol. 64, pp. 843-864.

Landes K.K. (1933) Origin and classification of pegmatites. American Mineralogist, Vol. 18, pp. 33-56, 95- 103.

Norton J.J. (1970) Composition of a pegmatite, Keystone, South Dakota. American Mineralogist, Vol. 55, pp. 98 1 - 1002.

Norton J.J. (1983) Sequence of mineral assemblages in differen- tiated granitic pegmatites. Economic Geology, Vol. 78, pp. 854-874.

Rickwood P.C. (198 1) The largest crystals. American Mineral- ogist, Vol. 66, pp. 885-907.

SchneiderhOhn H. (1961) Die Erzlagerstiitlen cler Etde. II. Die Pegmatite. Gustav Fischer Verlag, Stuttgart.

Simmons W.B. Jr., Heinrich E.W. (1980) Rare-earth pegmatites of the South Platte district, Colorado. Resource Series 1 1 , Colorado Geological Survey, Denver, CO.

Sinkankas J. (1970) Prospecting, for Gemstones and Minerals. Van Nostrand Reinhold, New York, NY.

Sinkankas J. (1981) Emerald and Other Beryls. Chilton Book Co., Radnor, PA.

Tuttle O.F., Bowen N.L. (1958) Origin o f Granite in Light of Experirnental Studies in the System NaAISiiOx-I<AISiiOx- SOy-HnO). Geological Society of America, Memoir 74.

Vlasov K.A. (1961) Principles of classifying granite pegmatites and their textural-paragenetic types. A/<adeiniya Nauk SSSR, Izvestiya, Seriya Geologicheskaya, pp. 5-20.

SUGGESTED ADDITIONAL READING Hurlbut C.S. Jr. (1968)Minerals andMan. RandomHouse, New

York, NY. Scalisi P., Cook D. (1983) Classic Mineral Localities of the

World-Asia and Australia. Van Nostrand Reinhold, New York, NY.

Sinkankas J. (1959) Gemstones of North America, Vol. I . Van Nostrand Reinhold, Ncw York, NY.

Sinkankas J. (l964)Mineralogy for Amateurs. D. VanNostrand, New York, NY.

Sinkankas J , (1976) Gemstones of North America, Vol. 11. Van Nostrand Reinhold, New York, NY.

Webster R., rev. by Anderson B.W. (1983) Gems, Their Sources, Descriptions, and Identification, 4th ed. Butterworths, London.

Gem-Bearing Pegmati tes GEMS & GEMOLOGY Summer 1984 77

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